Power instruments are known in which a reciprocating striker acts upon a head so that the head may produce an impacting force. The head itself may constitute the impacting tool, as in the case of a powered hammer, or it may serve to receive other tools, such as a chisel, star drill, or the like.
In U.S. Pat. No. 1,310,574, for example, an automatic spring-actuated hammer is described in which a spring is periodically put under tension by means of an electric motor, and periodically released in order to impart to the hammer an impact pressure corresponding to the force stored up in the spring by its previous tensioning.
A powered impact instrument is also described in U.S. Pat. No. 3,788,403, which issued Jan. 29, 1974 to the present inventor. The impact instrument of the present invention is similar in some respects to the mechanism described in U.S. Pat. No. 3,788,403, and is one in which a head is struck repeatedly by a spring-loaded striker. The striker is periodically drawn back against the resilient pressure of a main spring and released against the head, so that sufficient impact force may be developed at the head to drive nails, or the like, without the need for an excessively large mechanism.
The impact instrument to be described, like the instrument described in U.S. Pat. No. 3,788,403, is advantageous in that the stroke displacement of the striker, and the resulting impact pressure of the head, may be controlled by the operator. In this way, at the beginning of an operation, the head may be driven at a relatively low impact force, to start a nail, for example. Then, the instrument may be controlled so that the head is driven at an increased impact force to drive the nail home. When the instrument is used to drive a star drill, chisel, or the like, as for example, in the sculpting of stone or wood, the magnitude of impact is readily controllable to suit the particular application.